Current limited transistor switch

ABSTRACT

A CURRENT LIMITED TRANSISTOR SWITCH PROVIDING SWITCHING ACTION BETWEEN A SOURCE AND A LOAD IN RESPONSE TO TURN-ON AND TURN-OFF SIGNALS, AND PROVIDING CURRENT THRESHOLD SENSING FOR AUTOMATIC SWITCHING TO THE OFF CONDITION WHEN DESATURATION OCCURS. CHOPPERS, INVERTERS AND CIRCUIT BREAKERS INCORPORATING A CURRENT LIMITED TRANSISTOR SWITCH.   D R A W I N G

March 20, 1973 w. E. RIPPEL 3,721,836

CURRENT LIMITED TRANSISTOR SWITCH Filed Nov. 24, 1971 5 Sheets-Sheet 2FIG. 3.. fi 4 72/665? S/GWAL .5

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SWITCH CUREEA/TZ E756 .Era. 5c 1.0/30 CURRENT 22 THRESHOLD CUQEE/VTMarch 20, 1973 v w, mPPEL 3,721,836

CURRENT LIMITED TRANSISTOR SWITCH Filed Nov. 24, 1971 5 Sheets-Sheet 3m/aaexe s/aA/AL 3 02m S/GA/AL 4 1376. 76.

.2786. 76 5W/7'CH CUERENTZ 771EE5H0LD cue/25,117 W March 20, 1973 w. E.RIPPEL 3,721,836

CURRENT LIMITED TRANSISTOR SWITCH Filed Nov. 24, 1971 v 5 Sheets-Sheet 4:2, Q 63 FIG. 10.

17111212. y; ZQg '5 '6 A 65 5 z 33 Q 6 %"234 Z 4 /5 'March 20, 1973 w.E. RIPPEL 3 CURRENT LIMITED TRANSISTOR SWITCH Filed Nov. 24, 1971 5Sheets-Sheet La I.

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Patented Mar. 20, 1973 3,721,836 CURRENT LIMITED TRANSISTOR SWITCH WallyE. Rippel, 5781 Valley Oak Drive, Hollywood, Calif. 90068 Filed Nov. 24,1971, Ser. No. 201,671 Int. Cl. H03k 17/6'0 US. Cl. 307-253 22 ClaimsABSTRACT OF THE DISCLOSURE A current limited transistor switch providingswitching action between a source and a load in response to turn-on andturn-off signals, and providing current threshold sensing for automaticswitching to the off condition when desaturation occurs. Choppers,inverters and circuit breakers incorporating a current limitedtransistor switch.

BACKGROUND OF THE INVENTION This invention relates to a new and improvedtransistor switching circuit and to choppers, regulators, inverters,circuit breakers and the like incorporating transistor switchingcircuits. The invention is particularly directed to a new and improvedcurrent limited transistor switch which can be turned on and off asdesired and which always conducts in the saturation condition and whichwill automatically turn off if a desaturation condition develops.

A transistor is a three terminal solid-state device, the collectorterminal current (1 of which is a joint function of the base terminalcurrent (l and the voltage between the collector and emitter terminals(V (see FIG. 1).

A line V' divides the base current characteristic curves into twooperating regions. The area to the right of this line is referred to asthe active region, while area enclosed between the line and the L, axisis referred to as the saturated region. The distance between V and the Iaxis is called the collector to emitter saturation voltage (V and is afunction of I Typical values of V range between .05 volt and 1.5 volts.

In cases where the transistor is to be used as a linear or semi-linearamplifier, the active region characteristics are of importance.Conversely, in digital and power control applications, where thetransistor is to perform an on-off or switching action, the saturatedregion characteristics are of prime importance, since they correspond tothe onstate of the transistor. The off-state, it will be noted, isattained by simply making 1 :0. The transistor may be used as acurrent-controlled switch, where switching action takes place betweenthe emitter and collector terminals, and is controlled by action of thebase terminal current.

When operating the transistor in the on-state, a sufficiently largevalue of I must be present to insure saturation (V V Accordingly, for agiven value of base current, collector current must not exceed a certaincritical limit, lest the transistor desaturate.

There exist a large spectrum of applications where transistors are usedin the switching modes. Switching applications may further be dividedinto two sub-classes, namely digital signal processing and power controlapplications. With signal processing, transistor current and powerlevels are generally small compared to maximum ratings and accordingly,there is little concern about energy factors such as second breakdown,thermal runaway, excessive junction heating and the like.

Conversely, in power control applications, the story is quite differentand the above factors become most vital. For example, in many chopperand inverter applications, should the collector current become largerthan a certain level, desaturation will occur and within a fewmilliseconds, the transistor will be thermally destroyed.

In recent years, due mainly to advances in transistor fabricationtechniques, a tremendous number of power control applications have comeinto practice where power transistors are used as switches. For example,in the data processing industry, power transistors are used in theswitching mode to turn on and off display lights and to activatesolenoid drivers used in printers and key punches. In theinstrumentation area, power transistors, operated in the switching mode,are used for all sorts of voltage and current regulated power supplies.In areas where portable AC power is required, switching mode transistorcircuits convert DC. from batteries to AC. at a desired frequency. And,in a wide variety of applications, ranging from portable electric handtools to electrically driven vehicles, transistors, used in theswitching mode, chop the battery power for efiicient control of energyflow from the battery to the electric motor. Other applications wheretransistors are operated in the switching mode to effect power controlinclude bidirectional choppers, controlled rectifier arrays,cyclo-inverters, cyclo-choppers, crowbars, and electronic circuitbreakers.

In virtually all of the above power applications, three problems arise.On one hand, should the switching currents, even momentarily, exceed acertain critical limit, full turn-on of the switching transistor willnot be achieved which in turn will result in both a loss of energyconversion efiiciency and increased transistor dissipation, the latterof which is generally destructive to the switching transistor.

The second problem is a result of the remedy to the first problem. In anattempt to insure full saturation under worse case conditions, basedrive currents considerably in excess of those actually required aresupplied. Since all of the energy delivered to the base-emitter junctionends up as heat on the junction, it follows that the above practicerepresents a lower than optimal efficiency state, especially since inmost cases, this same high base drive current is used even when theswitching currents may be relatively small.

Problem three is that when a fault condition occurs, a combination ofdesaturation and high current conditions will prevail which willinevitably cause rapid thermal destruction of the switching transistor.

In various applications, such as inverters and chopper regulators,protection schemes have been devised which either directly or indirectlyremove base drive in the event that load current exceeds a predeterminedvalue. Most of these schemes, however, employ complicated feedbackcircuits which increase size, weight and cost of the system.Furthermore, in all but the most complex schemes, base drive is set at afixed level which results in higher than needed base-emitter powerlosses, especially during other than full load operation.

SUMMARY OF THE INVENTION The current limited transistor switch includesa switching transistor connected between source and load ter minals anda drive control circuit for the base current. A current threshold levelis maintained by the drive control circuit which maintains conduction inthe switching transistor in the saturation condition. If the transistorstarts to desaturate, feedback through an on-off control circuit gatesthe base current olf. Base current is gated on by a turn-on triggerpulse and the switching transistor remains on if conducting in thesaturation condition.

The switch performs a function similar to conventional circuits where atransistor is used as a unidirectional switch, but with several featuresof improvement. Turnon action is initiated by a trigger input. A highspeed turn-off action automatically results when current through theswitching transistor becomes larger than a threshold level, which inturn is proportionate to a voltage supplied by a drive input.

As a result of the above features, the switch of the invention offersboth fail-safe protection to the power switching transistor, while alsoproviding current-controlled behavior when used in applications such aschopper regulators. The switch is universal in nature, and may be usedin a variety of equipments, including chopper regulators, D.C. to AC.inverters, controlled rectifier arrays, cyclo-inverters, cyclo-choppers,electronic circuit breakers, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a set of typical transistorcharacteristic curves;

FIG. 2 is a diagram of a switching apparatus incorpo rating a presentlypreferred embodiment of the invention;

FIG. 3 is a diagram of an AC. switch or circuit breaker incorporatingthe switch of FIG. 2.

FIG. 4 is a diagram of a current regulating D.C. chopper incorporatingthe switch of FIG. 2;

FIGS. 50, 5b and 5c are timing diagrams illustrating the operation ofthe chopper of FIG. 4;

FIG. 6 is a diagram of a voltage and current regulating D.C. chopperincorporating the switch of FIG. 2;

FIGS. 7a, 7b, and 7c are timing diagrams illustrating the operation ofthe chopper of FIG. 6;

FIG. 8 is a diagram of a voltage boosting chopper similar to that ofFIG. 4;

FIG. 9 is a diagram of a voltage changing chopper similar to those ofFIGS. 4 and 8;

FIG. 10 is a diagram of a current regulating poly D.C. chopperincorporating the switch of FIG. 2;

FIGS. 11, 12 and 13 are diagrams of three variations of bidirectionalchoppers incorporating the switch of FIG. 2;

FIG. 14 is a diagram of a bidirectional poly chopper similar to those ofFIGS. 10, 11, 12 and 13; and

FIGS. 15, 16 and 17 are diagrams of three variations of invertersincorporating the switch of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT The circuit of FIG. 2 includes acurrent limited transistor switch 20 connected between a source 21 and aload 22, with a diode 23 connected across the load. The circuit alsoincludes a base drive power supply 24, an input power supply 25, a driveinput voltage source 26, and a trigger input or turn-on pulse source 27.

The base drive power supply 24 is connected between a source terminal 1and a base drive terminal 5 of the switch 20. The input power supply 26is connected between a terminal 6 and the base drive terminal 5. Thedrive input voltage source 26 is connected between a drive inputterminal 4 and the terminal 5, and the trigger input is connectedbetween a turn-on pulse terminal 3 and the terminal 5. The source 21 isconnected to the source terminal 1 and the load 22 is connected to aload terminal 2.

The transistor switch 20 includes a switching transistor 30 with emitterand collector connected between the source terminal 1 and load terminal2. The transistor switch also includes a drive control circuit 31 and anon-oif control circuit 32. The drive control circuit 31 includes athreshold level unit 33 and a current control unit 34.

In the preferred embodiment illustrated, the current control unit 34includes a transistor 37 connected in series with a resistor 38 betweenthe base of the switching transistor 30 and the base drive terminal 5.

The preferred threshold level unit 33 as illustrated in FIG. 2 includesan operational amplifier 40 with its output connected through a resistor41 to the base of the current control unit transistor 37. The driveinput termi- 118.1 4 is connected as an input to the amplifier 40through resistor 42. A resistor 43 is connected between the input of theamplifier 40 and the junction between the transistor 37 and resistor 38to provide another input to the amplifier. The on-off control circuit 32is connected as an input to the amplifier 40 through a resistor 44.

The preferred form of the on-off control circuit 32 includes atransistor 47 with emitter connected to the base drive terminal 5through a resistor 48 and with the emitter connected to the sourceterminal 1 through a diode 49. The collector of the transistor 47 isconnected to the resistor 44 and the base is connected to a junctionpoint 50.

A resistor 51 is connected between the switching transistor 30 and ajunction point 50, to provide a turn-oil signal as will be describedbelow. A capacitor 52 is connected between the turn-on pulse terminal 3and the junction point 50 for transmitting a turn-on pulse, as will bedescribed below.

While specific polarities for voltages, transistors and diodes have beenindicated in the circuit of FIG. 2, it will be readily understood thatthose skilled in the art may change polarities as desired.

The base drive supply 24 supplies base drive requirements for theswitching transistor 30. For typical operation, power supply 24 runs asa constant voltage source on the order of 2 to 4 volts. The input powersupply 25 supplies operating power to the operational amplifier 40. Inthe case of a monopolar operational amplifier, as shown in FIG. 2, amonopolar power supply is used. If desired, a bipolar operationalamplifier may be used, in which case a suitable bipolar power supply isused. In either case, the power supply 25 supplies constant voltage(s),the actual value(s) of which depend on the operational amplifier. Withsupplies 24 and 25 energized, terminals 1 and 2 will be nonconductive,until the proper voltages have been applied between terminal pairs 3, 5and 4, 5.

In order to cause turn-on action between terminals 1 and 2, twoconditions must prevail. First, a non-zero, unidirectional voltage ofthe correct polarity, which may be constant or time varying, must beapplied between drive input terminal 4 and common or base drive terminal5. Next, a trigger pulse of the correct polarity, duration and magnitudemust be supplied between turn-on pulse terminal 3 and terminal 5.

During the time of the trigger pulse, current will flow betweenterminals 1 and 2, the magnitude of which is either limited by the load,or is limited by action of the switching transistor, in which case, thecurrent is approximately proportionate to the instantaneous value of thedrive voltage between terminals 4 and 5.

Upon completion of the trigger pulse, terminals 1 and 2 will remain in amutually conductive state if and only if saturation of the switchingtransistor was achieved during the time of the trigger pulse. If theswitching transistor failed to saturate during the time of the triggerpulse, the circuit will automatically revert to the off-state anderminals 1 and 2 will be mutually non-conductive, following the triggerpulse.

Should the case prevail Where the switching transistor attainssaturation during the trigger pulse, terminals 1 and 2 will remainmutually conductive until the current through terminals 1 and 2 exceedsa certain threshold which, in turn, is approximately proportionate tothe instantaneous voltage between terminals 4 and 5at which time, theswitching transistor will be rapidly turned off. The drive input voltageat terminal 4 may be reduced to effect turn-off.

A second mode of turn-off is also possible. If a voltage pulse ofsufficient magnitude and of the correct polarity (reverse polarity ofturn-on pulse) is applied between terminals 3 and 5, turn-oi? Willsubsequently follow and terminals 1 and 2 will revert to thenon-conductive state.

The switching transistor 30 receives base drive current Which issupplied by base drive supply 24 and is controlled by action of drivertransistor 37. Transistor 37 is in turn driven by the output ofoperational amplifier 40. Accordingly, when the output of amplifier 4t)swings sufiiciently positive, transistor 30 will be driven intosaturation.

For the moment, assume that transistor so is in saturation (i.e., itscollector to emitter voltage is less than 1 volt). Assuming that nocurrent is caused to flow in terminal 3, it then follows that the baseof silicon transistor 47 will be less than 1 volt negative with respectto the emitter of transistor 30. Next, we note that resistor 43 biasessilicon diode 49 into forward conduction, making the emitter oftransistor 47 about .7 volt with respect to the emitter of transistor30. From these relations, it follows that the base-emitter junction oftransistor 47 will be forward biased by no more than .3 volt. Hencetransistor 47 will be non-conductive and no current will flow throughresistor 44. Accordingly, the only currents that will effect theinverting input of amplifier 40 will be those through resistors 42 and43.

Resistor 38 serves as a current sensing resistor by producing a voltagedrop which is proportionate to the current through transistor 37. Byaction of resistors 42 and 43, amplifier 40 drives transistor 37 suchthat the current through resistor 38 (and hence the base current totransistor 30) is proportionate to the drive voltage applied betweenterminals 4 and 5.

If for any reason, transistor 30 desaturates and its collector toemitter voltage exceeds a certain amount (in this case about 1.3 to 1.4volts), the base-emitter voltage of transistor 47 rises to the turn-npoint thus causing collector current to flow through resistor 44. Thiscollector current is of such direction that it opposes current flowingthrough resistor 42. In particular, if resistor 44 is sufficiently smallcompared with resistor 42, the resulting current through resistor 44will completely turn oif amplifier 40 thus causing transistors 37 and 30to also turn off. It is therefore seen that base drive to the mainswitching transistor 30 is turned off by regenerative action when itscollector to emitter voltage exceeds a certain threshold.

Once regenerative action has caused turn-off, transistor 30 will remainin the off state until a voltage pulse of the correct polarity issupplied between terminals 3 and 5. The application of such a pulse,causes momentary diversion of the base drive of transistor 47. As aresult, transistor 47 switches oil and remains off for the duration ofthe trigger pulse, during which time amplifier 40, in conjunction withtransistor 37, causes a base drive to transistor 30 which isproportionate to the drive input voltage. If this base drive totransistor 30 is sufiiciently large, transistor 30 will saturate duringthe time of the trigger pulse whereupon transistor 30 will continue toreceive base drive upon termination of the trigger input pulse. Ifhowever, the base drive to transistor 30 is not suificient to enablesaturation, base drive to transistor 30 will be removed upon terminationof the input trigger pulse.

It should be noted that for the duration of the trigger pulse,transistor 30, if unsaturated, will possibly dissipate a high level ofpower--perhaps many times its continuous rating capability. It isimportant therefore that the duration of the trigger pulse be keptsufficiently short so that the thermal capacity of the junction oftransistor 30 can absorb the resulting thermal energy without excessiveheating. It should also be noted that the duration of the trigger signalmust be somewhat longer than the turn-on time of transistor 30. Thesetwo considerations give respective upper and lower bounds for theduration of the trigger signal. For most present day silicon switchingtransistors, having turn-on times of only a few microseconds, it turnsout that trigger pulse durations of around microseconds provide bothreliable turn-on and at the same time are sufiiciently short toguarantee low values of junction heating, even under conditions of faultcurrents and maximum base drive levels.

One of the key features of the disclosure that should be noted is thatswitching transistor 30 is used in both a conventional as well asunconventional way. The switching action it effects is conventional. Thecurrent sensing function it provides, however, is unconventional. Inessence, transistor 30 is used as a current sensor or more accurately asa current-threshold sensor. For a given level of base current, thecollector to emitter voltage remains small and nearly constant until acertain magnitude of collector current is reached (threshold current) atwhich time the collector to emitter voltage increases rapidly withincreasing collector current. For a wide range of base currents, theabove mentioned collector threshold current is nearly proportionate tothe base drive current.

In a more generalized consideration, the threshold unit 33 may be anonlinear, active circuit having a time dependent response. Input supplyvoltage is applied between terminals 6 and 5. The drive input is appliedbetween terminals 4 and 5. The shunt voltage signal, which isproportionate to the base drive current of transistor 30, is appliedbetween terminals 39 and 5. A gating signal, which when present, causesthe output to be zero, regardless of other input signals, is appliedbetween terminals 45 and 5. Finally, output of the unit 33 is betweenterminals 46 and 5. The above can be summarized mathematically as as fi4, 39, if 45 20 if i 0 where i, is the current through the j terminaland V, is the voltage between terminal j and terminal 5.

The function is restricted to those cases where 1' is increasing withrespect to V and decreasing with respect to V f is also constrained suchthat the resulting circuit response will be stable.

In a more generalized consideration, the on-oif control circuit 32 maybe a circuit wherein the output current of which at terminal 45 is anonlinear time dependent function of the collector to emitter voltage ofthe switching transistor 30. In summary,

f must be an increasing function with respect to V By using variousfunctions f and f various modified circuit responses may be achieved.For example, by adjusting the time dependence part of f various basedrive responses (stable and unstable) can be obtained from a givensignal applied to the drive input. And, with respect to f it is notedthat by introducing nonlinearities (e.g. Schmitt trigger action) andtime dependence where a'V /dt is taken into account, turn-off oftransistor 30 may be initiated by more general features of thecharacteristic curves of transistor 30.

By way of summary of operation, the voltage signal at the drive inputterminal 4 sets the current threshold level for the switching transistor30 through the threshold level unit 33 and current control unit 34 ofthe drive control circuit 31. A feedback to the input is provided fromterminal 39 through resistor 43.

The switching transistor 30 is turned on or switched into conduction bya turn-on pulse at terminal 3 which is coupled to the threshold levelunit 33 by the on-otf control circuit 32.

The switching transistor 39 may be turned off or switched tononconduction in two ways. One is by means of a turn-off pulse atterminal 3. The other is by means of feedback from the transistor St tothe on-off control circuit 32 through resistor 51. Turn-off occursautomatically when the switching transistor 30 desaturates. This mayoccur at any time during operation of the circuit and serves a safetyfunction preventing damage to the transistor. This may be caused tooccur by varying the drive input voltage at terminal 4.

Various apparatus incorporating the current limited transistor switchare possible and several embodiments are described hereinbelow.

ON-OFF D.C. SWITCH In the circuit of FIG. 2, the current limitedtransistor switch 20 may be used as a voltage controlled switch wherebythe load 22 may be connected or disconnected from the DC power source21, the voltage of which may be either constant or time varying. Thetrigger input 27 is a time dependent voltage source which is used totrigger on and may be used to trigger off the current limited transistorswitch by application of voltage pulses of correct wave shapes to theterminal 3. The drive input 26 may be a battery which provides aconstant DC. voltage of the correct polarity and of suflicient magnitudeto the drive input, so that after trigger source 27 has initiated theturn-on of the transistor switch, it will not revert to the oil? state,until an appropriate off pulse has been generated by voltage source 27.The diode 23 need be in cluded in only those cases where the load 22 isinductive, in which case the diode serves as a bypass path forinductively driven currents which persist after switching transistor 30has been turned off.

The behavior of the on-otf D.C. switch circuit of FIG. 2 is such that:

(1) The transistor switch will connect the load 22 across the voltagesource 21 upon an appropriate com mand from trigger source 27. Assumingthat the load current remains sufficiently small, for a given value ofbattery 26 voltage, continuity will remain indefinitely.

(2) The transistor switch will revert to the off-state upon anappropriate command from trigger source 27.

(3) The transistor switch will revert to the off-state if battery 26voltage is reduced below a certain threshold which is roughlyproportionate to the load current.

D.C. CIRCUIT BREAKER The circuit of FIG. 2 also may serve as a DC.circuit breaker which has an adjustable trip point and is reset by theapplication of a voltage signal.

In particular, it will be noted that the magnitude of battery 26 voltageregulates the threshold value of current at which the current limitedtransistor switch 20 will revert to the oft-state. Accordingly, bymaking voltage source 26 an adjustable voltage source, it is possible toregulate the threshold point at which reversion to the off-state willoccur. In all cases, trigger source 27 provides turn-on or reset. Aswith the on-oft switch application, trigger source 27 may also be usedto turn off the transistor switch.

In the case where the trigger input 27 is able to provide recurrentturn-on pulses, an automatic reset action of the previously describedcircuit is possible. For example, if the trigger input 27 providespulses per second, of the correct wave shape, automatic reset will occurwithin .1 second after the time of fault removal.

It should be noted that the speed of circuit breaking action is limitedonly by switching speed limitations of the semiconductor devices used inthe current limited transistor switch. Accordingly, durations of lessthan one microsecond between the occurrence of a fault load and the timeof turn-off are possible. This extremely fast response time makes theabove mentioned electronic circuit breaker especially valuable whereprotection of solid-state equipment is involved.

ON-OPF A.C. SWITCH In the circuit of FIG. 3, the current limitedtransistor switch is used in a voltage-controlled switch, whereby a load22 may be connected or disconnected from an A.C. power source 21.Throughout the figures of the drawings corresponding elements areidentified by the same reference numerals. In FIG. 3 and succeedingfigures, the power supplies 24 and 25 are omitted in order to simplifythe figures, but of course they would be utilized in 8 each embodiment,connected to terminals 1 and 5, and 6 and 5, respectively. Diodes 5760are connected as a full Wave rectifier.

In explaining the theory of operation of the A.C. switch of FIG. 3, itis noted that there are four instantaneous conditions of state. In case1, the upper terminal of source 21 is positive with respect to the lowerterminal and switch 20 is in the non-conductive state. Case 2 is thesame as case 1 but source 21 has reversed polarity. Case 3 is the sameas case 1, except that switch 20 is in the conductive state. In case 4,the upper terminal of source 21 is negative with respect to its lowerterminal and switch 20 is in the on-state.

Assuming no load E.M.F., as with a resistive load, it is noted that noload current flows in either case 1 or 2, and that in both these cases,terminal 1 of the switch is positive with respect to switch terminal 2.Hence, when switch 20 is in the off-state, the load is effectivelydisconnected from the A.C. power source.

In case 3, current will fiow from the upper terminal of source 21,through diode 58, through transistor switch 20, through diode 59, andthrough load 22 to the lower terminal of the source 21. Neglecting thevoltage drops of diodes 58 and 19 and neglecting the voltage drop oftransistor switch 20, it is seen that load 22 is effectively connectedacross source 21 in this case. In like manner, it will be noted thatload 22 is also connected across source 21 in case 4.

As a result of the analyses of cases 1 through 4, it is seen that thetransistor switch in FIG. 3 can serve to effectively connect anddisconnect load 22 from an A.C. source 21.

As in the previous cases, switch 20 is turned on by action of a voltagepulse from trigger source 27 and turnoff is initiated by eitherdecreasing the voltage between terminals 4 and 5, or by providing areverse voltage pulse between terminals 3 and 5.

A.C. CIRCUIT BREAKER The circuit of FIG. 3 may also serve as an A.C.circuit breaker which has adjustable peak current trip point and isreset by application of a voltage signal.

It will be noted that load current, whether positive or negative, mustflow through switch 20. Accordingly, whenever either the positive or thenegative peak of the load current exceeds a certain threshold, which isapproximately proportionate to the voltage of drive input voltage source26, the current limited transistor switch 20 reverts to the off-state,thus carrying out the action of an A.C. circuit breaker.

Reset action is provided by the application of a pulse voltage of thecorrect shape between terminals 3 and 5. As with the DC. circuit breakerapplication, a source of repetitive voltage pulse may be used for thetrigger input 27 thus enabling automatic reset.

CURRENT REGULATING D.C. CHOPER In many applications, especially whereDC. motors are used, a control device is required which provideslossless energy conversion where energy is obtained from a source ofconstant or nearly constant voltage and supplied to a load, thecharacteristics of which vary with time. Using the current limitedtransistor switch 20, it is possible to obtain a circuit which providesa near lossless transfer of energy from source to load and one whereload current remains regulated at a value which is approximatelyproportionate to an input control voltage. One such circuit is shown inFIG. 4.

The DC. power source 21 may be constant or time varying. The triggerinput 27 is a source of recurrent trigger pulses which are capable oftriggering transistor switch 20 into conduction. The diode 2-3 is theconventional free-wheel diode and inductor 63 is a load currentsmoothing inductor.

The operation is as follows. Assume that drive input voltage source 26is held at a constant value and that trigger source 27 supplies a trainof turn-on trigger pulses (FIG. a). Furthermore, assume that voltage 26is set at a sufficiently low value such that the transistor switchcannot maintain the resulting steady-state load current withoutreverting to the off-state.

Initially assume the load current (FIG. 5c) is zero. When the firsttrigger pulse is applied to terminal 3, the transistor switch willswitch on and will remain on until the load current reaches thethreshold level, (FIG. 5b) which in turn is determined by the magnitudeof voltage 26 at terminal 4. The time required until this thresholdlevel is reached will of course be determined by the value of theinductor 63 and parameters of the load 35.

Directly after switch reverts to the off-state, load current will flowin a circular path through free-wheel diode 23. The rate of decay ofthis current will be determined by the ratio of the resistance toinductance of load 22 and inductor 63.

Eventually, after a time interval, which is long compared to the abovementioned time constant of the load, a steady-state condition will beattained. Under this condition, switch 20 will be turned on with eachtrigger pulse; load current will rise to the threshold point betweentrigger pulses and hence turn-off will also occur between successivetrigger pulses. As a result, source and load current wave-forms as shownin FIGS. 5b and 5c, respectively, will prevail.

In the case where the load time constant is long compared with theperiod between successive trigger pulses, the ripple component of theload current will be small compared with the DC. component of the loadcurrent. In this case, the DC. load current component will be nearlyequal to the peak load current. Since the peak current and the turn-offthreshold current are the same, it follows that in this case, the DC.load current will be proportionate to the drive voltage supplied bysource 26. In effect then, the circuit of FIG. 4 provides a losslesstransfer of energy from voltage source 21 to load 22 such that the D.C.component of load current is maintained nearly proportionate to thedrive voltage applied between terminals 4 and 5. Of course, in reality,some small losses will occur which are the sum of switch losses, diode23 losses, and losses due to resistance associated with inductor '63).

In addition to the features already mentioned, the circuit of FIG. 4 hasa number of other very advantageous characteristics. First of all,should the load become shorted, the action of transistor switch 20 issuch that the resulting currents will remain at safe values and nodamage will occur to any of the circuit components. Simi larly, there isno danger to any of the circuit components should load 22 generatetransient load changes during the course of operation.

Next, it will be noted that the base drive power drawn from supply 24 toterminals 1 and 5 (see FIG. 2) is proportionate to the magnitude ofdrive voltage 26. Hence, under low current conditions, where drivevoltage 26 is adjusted to a relatively small value, the power drawn fromsupply 24 is also small and hence the overall circuit eificiency remainshigh.

Another important point of the circuit of FIG. 4 is its extremesimplicity. With conventional chopper schemes, a current sensing circuitplus a duty cycle generator would be required to effect the same actionof current limiting. That the FIG. 4 circuit does not require thesecomponents means both an improvement in reliability and a reduction inweight, size and expense.

A number of useful modifications of the FIG. 4 circuit may be made. Forexample, the recurrent trigger pulse source 27 may be replaced with asimilar pulse source, the frequency of which, rather than beingconstant, is made a function of drive voltage 26. With thismodification, it is possible to further optimize energy conversionefliciency over a wider range of operating conditions. A second usefulmodification is to replace the D.-C. voltage source 26 with a D.-C. timevarying voltage source. In the case where the modified voltage sourcegenerates a voltage which is periodic and has the same period as thetrigger pulses from source 27, various voltage-current relations, inaddition to the previously described constant current case, can beattained. a third modification would be the inclusion of a more advancedoutput filter, which could be connected between the output of inductor63 and the load 22.

VOLTAGE REGULATING-CURRENT REGULATING D.C. CHOPPER In many applications,a control device is required which provides lossless energy conversionwhere energy is obtained from a source of constant or nearly constantvoltage and supplied to a load in such a way that either the voltageacross the load will be held at a determined value or the currentthrough the load will be held at a determined value.

The current limited transistor switch 20 may be utilized in an apparatuswhereby the above action can be carried out. One such scheme is shown inFIG. 6 which is identical with the circuit of FIG. 4 with the exceptionthat generator 66 and potentiometer 67 are used for the drive input 26between terminals 4 and 5, with a feedback connection 68 from the loadto the generator 66. The generator 66 is a voltage-controlled duty-cyclegenerator which functions to produce an output signal as shown in FIG.7b, where the generator output pulses at terminal 4 are synchronizedwith the trigger pulses at terminal 3, and where the duty cycle isproportionate to the difference between the actual output voltage and adesired voltage level (reference).

In those cases where the output current is sufiiciently low, compared tothe drive signal applied to terminal 4, the on-period of switch 20 willcorrespond exactly with the on-period of the drive voltage applied toterminal 4. In this case, a condition of output voltage regulation willexist and the load voltage will be maintained very nearly equal to thereference voltage (see FIG. 7c).

However, in those cases Where the switch current reaches the thresholdvalue within the period of the duty cycle of generator 66, circuitoperation will be identical with the circuit of FIG. 4. Accordingly, acondition of current control will take place and the load voltage will,in general, be significantly below the reference level. This in turnwill cause the duty cycle of geneartor 66 to attain its maximum value.Accordingly, turn-off of switch 20 will no longer be initiated by theduty cycle generator, but rather by action of the current thresholdeffect inherent in the current limited transistor switch itself. In thismode of operation, it will be noted that the drive signal to inputterminal 4 is proportionate to the setting of potentiometer 67 which isconnected across the output of the generator 66. It therefore followsthat the current limit is regulated in proportion to the setting ofpotentiometer 67.

A modification of the apparatus is to have the duty cycle generator actthrough the trigger input, rather than through the drive input, sinceturn-off of switch 20 can be accomplished by applying a pulse of thecorrect polarity (reverse of the turn-on pulse) to the trigger input. Inthis case, the drive input would simply be connected to an adjustableD.-C. voltage source, as in FIG. 4. Other modifications, similar tothose discussed in connection with the FIG. 4 circuit are possible.

VOLTAGE BOOSTING CURRENT REGULATING D.C. CHOPPER In the apparatus ofFIGS. 4 and 6, transistor switch 20, diode 23 and inductor 63 jointogether and form a three terminal network A, B, C. In the mostfrequently used case, the diode 23 is common to both the input 21 andoutput 22 of the chopper, as in FIGS. 4 and 6. However, the same networkof elements 23, 63, and 20 can be applied usefully where either theinductor 63 or the transistor switch 20 terminals are common to bothinput and output.

In FIG. 8 and succeeding figures, the turn-on pulse source 27 and thedrive input voltage source 26 are omitted in order to simplify thefigures, but of course they would be utilized in each embodiment,connected to terminals 3 and 5, and 4 and 5, respectively.

In the circuit of FIG. 8, the interconnection between elements 23, 63and 20 is indeed the same as with FIGS. 4 and 6. However, in FIG. 8,terminal 1 of the transistor switch is common to both the power source21 and the load 22, and a voltage step-up action between the source andthe load is possible. In particular, it is noted that as the duty cycleof switch 20 is increased from zero to unity, the effective step-upratio between the load voltage and the source voltage increases, rangingbetween unity and an unbounded limit.

In the case where a recurrent source of voltage pulses is appliedbetween terminals 3 and 5, and where a fixed source of D.-C. voltage isapplied between terminals 4 and 5, energy will be removed from voltagesource 21 and delivered to load 22 in such a way that the averagecurrent drawn from source 21 will remain proportionate to the D.-C.drive voltage applied between terminals 4 and 5. Furthermore, thecurrent drawn from source 21 will remain essentially constant, withrespect to changes in: source 21 voltage, load 22 impedance, and load 22E.M.F. The circuit just described may be modified in ways exactlyanalogous to the modifications discussed in association with FIGS. 4 and6.

VOLTAGE CHANGING CURRENT REGULATING D.C. CHOPPER The apparatus of FIG. 9provides a third way in which the basic circuit consisting of switch 20,diode 23, and inductor 63 :may be connected to the source and load, toproduce a useful effect. The unique feature of the circuit of FIG. 9 isthat energy may efficiently be transferred from a D.-C. voltage source21 to a load 22 regardless of the load In both the cases where the loadis greater or less than the source E.M.F., efficient energy transfer ispossible. In the case where load 22 is resistive, the load voltage maybe controlled to any value whatsoever, within the limitations of thecircuit component ratings. Thus, this circuit may be used to produceeither a step-up or a step-down action.

As with the FIG. 8 circuit, a source of voltage pulses is connectedbetween terminals 3 and 5 and an adjustable D.-C. voltage is connectedbetween terminals 4 and 5. The action of these two voltage sources isvirtually the same as with the FIG. 8 circuit.

In the circuit of FIG. 9, the average current drawn from source 21 is ajoint function of both the magnitude of the voltage applied to the driveinput, and the parameters of the load. Both the peak source and peakload currents are limited (and equal to) the threshold current of thecurrent limited transistor switch; this threshold current in turn isproportionate to the drive input voltage.

This circuit may be modified in ways exactly analogous to themodifications discussed in association with FIGS. 4 and 6.

CURRENT REGULATING DUAL D.C. CHOPPER One of the disadvantages of usingchoppers as a means of D.-C. power control is that the chopping actionintroduces undesired A.-C. current harmonics into both the source andload circuits. While filter circuits can be used to reduce the contentof these current harmonics, such filters add greatly to the weight andcost of the chopper system and also introduce losses of their own.

Chopper circuits have been developed in recent years which greatlyreduces the above mentioned current harmonics without the addition offilter circuits. These are sometimes referred to as dual choppers andpoly choppers,

12 where two or a plurality of switches are used. Such chopper circuitsare described in:

(1) Three-Phase Silicon Controlled Rectifier Battery Charger-by Wally E.Rippel-IEEE Region 6 Conference-ProceedingsI'EEE Resources RoundupApril,1969.

(2) A High Performance Electric Vehicle Control SystemMasters Thesis byWally E. Rippel-published with Cornell Universitys School of ElectricalEngineering1971.

(3) Dual SCR Chopper as a Motor Controller for an Electric Carby WallyE. RippelIEEE Transactions on Vehicular Technology, vol. VT-20, No. 2May, 1971.

A poly chopper circuit is illustrated in FIG. 10 with the three terminalnetwork A, B, C designation of FIGS. 4, 8 and 9. The source 21 and load22 may be connected as in FIG. 4 or as in FIG. 8 or as in FIG. 9.Consider first the operation as a dual chopper with two current limitedtransistor switches 20, 20. The duty cycles of switch 20' may be causedto lag (or lead) the duty cycles of switch 20 by exactly one halfswitching period by controlling the timing of the turn-on pulses atterminals 3. Then all of the odd current harmonics induced in bothsource 21 and load 22 which result from the switching action of switch20 are completely cancalled by the corresponding odd current harmonicsgenerated by action of switch 20'. Because of this odd harmoniccancellation, the r.m.s. content of current harmonics delivered to theload is reduced by more than a factor of 10 for typical circuitparameters, while the input r.m.s. harmonic content is approximately cutin half, for typical circuit parameters.

If current limited transistor switches are used in place of conventionalelectronic switches, a number of improvements result. First of all, thecircuit of FIG. 10 has all the advantages of the circuit of FIG. 4 plusthe features of reduced harmonic content.

Also of importance is the fact that, in the current regulating mode, anautomatic effect of load sharing takes place between the switchingtransistor of switch 20 and the switching transistor of switch 20'.Furthermore, because of automatic turn-ofi of both switches, loadsharing continues in the event of a component failure (e.g., diode 23 orinductor 63).

It will further be noted that dual chopper equivalents of the circuitsof FIGS. 4, 6, 8 and 9 are possible (including dual chopper equivalentsof their corresponding modifications), and that with each of these dualchopper equivalents, all of the previously described circuitcharacteristics remain, but with the added features of reduced currentharmonics.

CURRENT REG'ULATING POLY D.C. CHOPPER The dual chopper technique relatedmay be extended to any number of switches in the poly chopper, and threeare shown in FIG. 1(), namely 20, 20', 20".

By providing the proper phase delays between the various switches of thepoly chopper, current harmonic contents at terminals A, B, and C can befurther reduced relative to the corresponding terminal currents of thedual chopper circuit. It should be noted that as the number of choppingelements is increased, harmonic contents continue to decrease.

As with the dual chopper circuit, the current limited transistor switchalso finds favorable application with the poly chopper circuit and withits various modes of connection and modification.

BIDIRECTIONAL CHOPPER APPLICATIONS By combining the circuit of FIG. 4with the circuit of FIG. 8, a bidirectional control element resultswhich is capable of efficiently transferring D.-C. energy from a voltagesource 21 to a load 22, and in the case where the load possesses anE.M.F., energy can also be efliciently transferred from the load back tothe source. Such an apparatus is shown in FIG. 11.

Enregy transfer from source 21 to load 22 is effected by activatingswitch 20, while keeping switch 20a in the off-state. Conversely, energytransfer from load 22 to the source 21 is eifected by activating switch20a, while keeping switch 20 in the offstate.

The current limited transistor switch may be directly applied to thecircuit of FIG. 11 and is connected as indicated and all of thepreviously mentioned advantages of the current limited transistor switchcircuit are retained. One particular advantage of the invention relevantto the application of FIG. 11 is that in the event simultaneous triggerpulses are supplied to switches 20 and 20a, the resulting fault currentsthat normally would flow through switches 20 and 20a, are prevented bythe inherent turn-off action of the current limited transistor switches.

Circuit analysis of FIG. 11 reveals that the of load 22 must be lessthan the of source 21, if proper operation is to occur. In cases wherethe load is always greater than the source E.M.F., and bidirectionalenergy control is desired, a modification of the FIG. 11 circuit resultsin the circuit of FIG. 12. Note that in both circuits, switches 20 and20a, diodes 23 and 23a and inductor 63 are identically interconnectedforming a three terminal network ABC.

The current limited transistor switch finds application in a thirdversion of the bidirectional chopper, with FIG. 13 being a bidirectionalversion of the FIG. 9 circuit. The circuit of FIG. 12 is capable oftransferring energy, in either direction between voltage source 21 andload 22, where the load may be either greater than, equal to, or lessthan the of the source.

The modifications and performance features associated with the circuitsdepicted in FIGS. 4, 6, 8, and 9 are applicable to the circuits of FIGS.11, 12 and 13.

By combining the principles of the bidirectional chopper and the polychopper, a composite three terminal chopper circuit is obtained whichmay be connected in arrangements analogous to the circuits of FIGS. andFIGS. 11, 12, and 13, and such a circuit is shown in FIG. 14. Theoperation will be as described in conjunction with FIGS. 10-13.

INVERTER APPLICATIONS The current limited transistor switch may beutilized in various inverter circuits, wherein D.-C. energy istransformed to A.-C. enregy. The application of the current limitedtransistor switch to three inverter circuits is shown inFIGS. 15,16 and17.

With the inverter of FIG. 15, switches 20 and 20b may be current limitedtransistor switches as described above. The operation of the invertercircuit entails making switches 20 and 20b alternately conductive at afrequency equal to the desired frequency of inversion, by means ofappropriately timed pulses at terminals 3. Diodes 71 and 72 providereturn paths for reactive currents which result from transformer andload reactance.

In the conventional inverter connected as shown in FIG. 13, thetransistors used for the switches 20 and 2% receive base drive eitherfrom an external circuit (case 1), or from auxiliary windings includedwith transformer 73 (case 2).

In the case 1, where base drive is obtained from an external circuit,precise, frequency control and in some instances, a certain degree ofoutput voltage control is possible. In this case, however, overcurrentprotection of the switching transistors and/or control of the loadcurrent require complicated and expensive auxiliary circuits.

In case 2, where base drive is obtained from transformer 73, extremecircuit simplicity plus a certain degree of overcurrent protectionresults. There are, however, a large number of disadvantages whichresult in this second case, among which are lack of accurate frequencycontrol, lack of voltage control, and lack of current control.

If current limited transistor switches are used in place of conventionaltransistors, a number of advantages result, among which are high energyconversion efficiency, inherent component protection with respect toovercurrent conditions, voltage control capability, and current controlcapability.

Operational details are as follows: Switches 20 and Ztlb are caused toconduct alternately and at the desired frequency. In the case wherevoltage control is desired, both switches 20 and 2011 are allowed toconduct for less than one half cycle. By controlling the preciseintervals over which switches 20 and 20b conduct, precise control of theA.-C. output voltage is achieved. The actual implementation of thiscontrol is exactly analogous with the circuit of FIG. 6. Furthermore,the regenerative turn-off action of the current limited transistorswitches can be used to separately control both the positive and thenegative peak load currents. This action is caused simply by separatecontrol of the voltages applied between terminal pairs 3 and 5 of eachswitch.

In summary then, it will be noted that the inverter using currentlimiting transistor switches is capable of providing frequency control,voltage control, positive peak current control and negative peak currentcontrol.

A second type of inverter frequently used is the bridge inverter circuitshown in FIG. 16. In operation, switch pairs 20 and 2% are turned on andturned off simultaneously as are switch pairs 20b and 20c. Diodes 71,72, 742, 75 return reactive energy from the load 22 to the DC. energysource 21. Frequency, voltage, and current control can be attained inways exactly analogous with the circuit of FIG. 15. Since switches 20and 20d are required to turn on and turn off simultaneously, the voltagesignals applied between terminals 3 and 5 must be equal, and the voltagesignals applied between terminals 4 and 5 must be equal for bothswitches. Similar voltage relations must prevail for switches Ztlb and200.

A third inverter where the current limited transistor switch may be usedis shown in FIG. 17. In this case, source 21 is a center tapped voltagesource (i.e., the voltage across 21a is equal to the voltage across21b). Switches 20 and 20b are operated with exactly the same constraintsas with the circuit of FIG. 15. Accordingly, frequency, voltage, andcurrent control are attained in ways exactly analogous with the FIG. 15inverter.

What is claimed is:

1. A current limited transistor switch for operation with a drive inputvoltage source, a base current source and a turn-on pulse source,comprising in combination:

a switching transistor having collector and emitter connected as aswitch between a source terminal and a load terminal;

a drive control circuit for the base current of said switchingtransistor for setting a current threshold level for said switchingtransistor varying as a function of a drive input voltage at a driveinput terminal of said drive control circuit,

said drive control circuit including a current control unit connectedbetween said switching transistor base and a base drive terminal forcontrolling the current of the base current source from said base driveterminal to said switching transistor base, and

a threshold level unit having said drive input voltage as an input and asignal varying as a function of the base current of said switchingtransistor as an input and providing an output in controlling relationto said current control unit for varying said switching transistor basecurrent as a function of said drive input voltage;

an on-off control circuit providing a gating signal as an input of saidthreshold level unit for changing said current threshold level as afunction of inputs to said on-otf control circuit to turn said switchingtransistor on and off;

first circuit means for connecting a turn-off signal vary- 15 ing as afunction of current through said switching transistor between saidsource and load terminals, to said on-off control circuit as an input toturn said switching transistor off blocking current between said sourceand load terminals; and second circuit means for connecting a turn-onpulse to said on-ofi control circuit as an input to turn said switchingtransistor on for current conduction between said source and loadterminals.

2. Apparatus as defined in claim 1 including a resistor in seriesbetween said current control unit and said base drive terminal, withsaid signal varying as a function of the base current of said switchingtransistor being developed across said resistor.

3. Apparatus as defined in claim 1 wherein said current control unitincludes a current control transistor with emitter and collectorconnected between said switching transistor base and said base driveterminal, and with said output of said threshold level unit connected tothe base of said current control transistor.

4. Apparatus as defined in claim 1 wherein said thre hold level unitincludes an operational amplifier having a first resistor connectedbetween said drive input terminal and the amplifier input, a secondresistor connected between said current control circuit and saidamplifier input, and a third resistor connected between said onotfcontrol circuit and said amplifier input.

5. Apparatus as defined in claim 1 wherein said on-off control circuitincludes a gating transistor providing current and no-current signals tosaid threshold level unit input, with said gating transistor controlledby signals at its base.

6. Apparatus as defined in claim 5 wherein said turnoff signal isdeveloped across said switching transistor collector and emitter, andsaid first circuit means includes a resistor connected between said loadterminal and said gating transistor base.

7. Apparatus as defined in claim 6 wherein said second circuit meansincludes a capacitor connected between said gating transistor base and aturn-on pulse terminal for coupling to said turn-on pulse source.

8. Apparatus as defined in claim 1 wherein said turnoff signal isdeveloped across said switching transistor collector and emitter, andsaid first circuit means includes a resistor connected between said loadterminal and said on-off control circuit input.

9. Apparatus as defined in claim 1 including:

a drive input voltage source connected at said drive input terminal, and

a turn-on pulse source connected at said second circuit means forproviding both positive going and negative going pulses for turning saidswitch on and ofi.

10. Apparatus as defined in claim 1 including:

an adjustable drive input voltage source connected at said drive inputterminal providing an adjustable switch opening point, and

a turn-on pulse source connected at said second circuit means forproviding recurring voltage turn-on pulses for resetting said switch tothe closed condition after opening.

11. Apparatus as defined in claim 1 including a full wave rectifierhaving opposed pairs of terminals, with one of said pairs of terminalsconnected across said switch source and load terminals and with theother of said pairs of rectifier terminals for connection to an A.C.source and a load.

12. Apparatus as defined in claim 1 including:

an inductor connected between said load terminal and an A terminal;

a diode connected between said load terminal and a B terminal, with saidsource terminal comprising a C terminal, forming a three-terminalnetwork ABC; a drive input voltage source connected at said drive inputterminal providing a drive input voltage of a value less than thatrequired for maintaining a steady state load current through saidswitch; and

a turn-on pulse source connected at said second circuit means providingrecurring voltage turn-0n pulses for repeatedly turning said switch on.

13. Apparatus as defined in claim 12 with a source connected betweenterminals C and B and with a. load connected between terminals A and B.

14. Apparatus as defined in claim 12 with a source connected betweenterminals A and C and a load connected between terminals B and C.

15. Apparatus as defined in claim 12 with a source connected betwenterminals C and A and with a load connected between terminals B and A.

16. Apparatus as defined in claim 1 including:

an inductor connected between said load terminal and an A terminal;

a diode connected between said load terminal and a B terminal, with saidsource terminal comprising a C terminal, forming a three-terminalnetwork ABC;

a drive input voltage source connected at said drive input terminalproviding a drive input voltage pulse of duration varying as a functionof the difference between a reference voltage and the voltage at theload;

a voltage feedback connection between the load and said drive inputvoltage source; and

a turn-on pulse source connected at said second circuit means providingrecurring voltage turn-on pulses in synchronism with said drive inputvoltage pulses.

17. Apparatus as defined in claim 1 including:

a plurality of said switches;

a corresponding plurality of inductors, with an inductor connectedbetween the load terminal of each switch and an A terminal;

a corresponding plurality of diodes, with a diode connected between theload terminal of each switch and a B terminal;

means connecting the source terminals of each switch to a C terminal,forming a three-terminal network ABC;

a drive input voltage source connected at the drive input terminal ofeach switch; and

a pulse source connected at the second circuit means of each switch andproviding recurring voltage turnon pulses for each of said switches.

18. Apparatus as defined in claim 1 including:

a second of said switches, with the load terminal of the first of saidswitches and the source terminal of the second of said switchesinterconnected at a first point;

a first diode connected between said first point and the source terminalof the first switch comprising terminal C;

a second diode connected between said first point and the load terminalof the second switch comprising terminal B;

an inductor connected between said first point and a terminal A, forminga three-terminal network ABC;

a drive input voltage source connected at the drive input terminal ofeach of said switches; and

a turn-on pulse source connected at the second circuit means of each ofsaid switches providing recurring voltage turn-on pulses alternately foreach of said switches.

19. Apparatus as defined in claim 1 including:

a second of said switches;

a transformer having primary and secondary windings;

a load connected across said secondary winding;

a first diode connected across the source and load terminals of thefirst of said switches;

a second diode connected across the source and load terminals of thesecond of said switches, with the source terminals of said switchesconnected to gether and with the load terminals of said switchesconnected across said primary winding; and

a source connected between said source terminals and the midpoint ofsaid primary winding.

20. Apparatus as defined in claim 1 including:

second, third and fourth switches corresponding to said first switch;

four diodes, with a diode connected across the source and load terminalsof each of said switches, respectively;

a source connected between the source terminals of said first and thirdswitches and load terminals of said second and fourth switches; and

a load connected between the load terminal of said first switch andsource terminal of said second switch and the load terminal of saidthird switch and source terminal of said fourth switch.

21. Apparatus as defined in claim 1 including:

a second of said switches;

a first diode connected across the source and load terminals of thefirst switch;

a second diode connected across the source and load terminals of thesecond switch, with the load terminal of the first switch and sourceterminal of the second switch interconnected at a first point;

a source connected between the source terminal of the first switch andthe load terminal of the second switch; and

a load connected between the midpoint of said source and said firstpoint.

22. Apparatus as defined in claim 1 including:

an inductor connected between said load terminal and an A terminal;

a diode connected between said load terminal and a B terminal, with saidsource terminal comprising a C terminal, forming a three-terminalnetwork ABC;

a drive input voltage source connected at said drive input terminal;

a pulse source providing both turn-on and turn-off pulses connected atsaid second circuit means, and providing a time delay between theturn-off and turnon pulses which is a function of the difference betweena reference voltage and the voltage at the load; and

a voltage feedback connection between the load and said pulse source.

References Cited UNITED STATES PATENTS 3,235,787 2/1966 Gordon et al307255 X 3,284,692 11/1966 Gautherin 307-297 X 3,364,391 1/1968 Jensen307255 X J. ZAZWORSKY, Primary Examiner US. Cl. X.R.

307237, 240, 255, 296; 31731, 33 VR; 321-11; 3239, DIG. 1

